What Are the Main Types of Osteosarcoma in Children?

Yes, there are several different types of osteosarcoma. When you read a pathology report, you will likely see two main ways the tumor is classified: by its histologic subtype (what the cells look like under a microscope and what kind of material they produce) and by its location relative to the bone (whether it started inside the bone or on the surface) ^[1][2].

It is completely normal to feel overwhelmed by the complex medical terms on these reports. While these specific classifications provide important details, the subtype alone does not tell the whole story. A child’s treatment plan and overall outlook also depend heavily on the tumor’s stage (whether it has spread, such as to the lungs), its grade (how aggressive the cells look), and whether the tumor can be completely removed with surgery ^[3][4].

Histologic Subtypes: What the Cells Look Like

Most childhood osteosarcomas are “conventional high-grade” tumors ^[1]. In pathology, high-grade does not just mean the cells divide quickly; it means the cells look very abnormal, are disorganized, and show high microscopic activity, indicating a more aggressive tumor ^[5][6].

Within this conventional group, pathologists categorize the tumor based on the type of matrix (connective material) the cancer cells are producing. The three most common conventional subtypes are:

• Osteoblastic: The cancer cells primarily produce large amounts of abnormal bone material, called osteoid ^[1].
• Chondroblastic: The cancer cells produce a cartilage-like material ^[1].
• Fibroblastic: The cancer cells look more like spindle-shaped connective tissue cells ^[1].

What this means for treatment: Seeing “osteoblastic” or “chondroblastic” on a report can be intimidating, but these three conventional high-grade subtypes generally share the same treatment backbone: systemic chemotherapy (drugs that travel throughout the body) given before and after surgery ^[7][2]. The specific subtype does not significantly change the overall prognosis ^[8]. Instead, care teams look closely at how well the tumor responds to the initial chemotherapy. This is measured after surgery by the pathologist, who checks the removed tumor tissue to see how much of it has died (a process called necrosis) ^[9][3].

There are also a few less common high-grade subtypes:

• Telangiectatic osteosarcoma: This type features blood-filled cysts and can sometimes be mistaken for a benign bone cyst on an X-ray ^[10]. Despite its unique appearance, it is generally managed with the same chemotherapy and surgery approach as other conventional high-grade tumors ^[11].
• Small-cell osteosarcoma: The cells in this rare subtype look very similar to Ewing sarcoma. Pathologists often need to run special molecular or genetic tests on the tumor tissue itself (these are not tests for an inherited genetic condition) to confirm the diagnosis and ensure the correct treatment plan is used ^[12].

(Note: This guide focuses on the most common pediatric subtypes. If you see a term not listed here, ask your child’s oncologist to explain it.)

Intramedullary vs. Surface Lesions: Where the Tumor Starts

Osteosarcoma is also classified by where it physically begins growing in the bone. This classification helps doctors decide the best surgical approach and whether chemotherapy is necessary.

Intramedullary (Central) Osteosarcoma

These tumors start inside the bone marrow cavity (the hollow center of the bone).

• Conventional high-grade osteosarcoma is intramedullary and is the most common type ^[2]. Because it is aggressive, standard treatment involves systemic chemotherapy and surgery ^[3].
• Low-grade central osteosarcoma is a much less common intramedullary type. Because it is low-grade, it is usually treated with wide surgical removal (taking out the tumor along with a healthy border of tissue) ^[13]. Chemotherapy is generally not needed unless the pathologist finds high-grade areas within the tumor.

Surface Osteosarcomas

These tumors grow on the outside surface (cortex) of the bone. They account for a smaller percentage of cases and are categorized by their grade:

• Parosteal osteosarcoma: This is a low-grade, slow-growing tumor on the bone’s surface. It is often treated with surgery alone, provided the surgeon can achieve clear margins (meaning no cancer cells are seen at the outer edge of the removed tissue) ^[14][13]. Chemotherapy is usually considered only if the tumor has “dedifferentiated” (developed high-grade, aggressive areas) or spread ^[15].
• Periosteal osteosarcoma: This is considered an intermediate-grade surface tumor. It requires surgery, but the decision to add chemotherapy is highly individualized based on the tumor’s specifics ^[16][14].
• High-grade surface osteosarcoma: This tumor grows on the surface but behaves aggressively, just like a conventional intramedullary tumor. Therefore, it is generally managed using the same approach: both chemotherapy and surgery ^[17][14].